Smart windows are glazing units that incorporate devices which have controllable optical and thermal transmission properties. The devices are generally in the form of layers either directly deposited on, or laminated to, the glass surface. Integration of the so-called smart windows into a building provides the opportunity to control internal light levels and temperature by adjusting the optical and thermal properties of the windows. Electrochromic devices, suspended particle devices (SPDs) and polymer dispersed liquid crystal (PDLC) devices are all examples of devices that are incorporated in smart windows; light transmission in these particular devices is electrically controllable, and smart windows incorporating these devices are also known as electrically tintable windows.
Electrochromic devices are currently incorporated in a range of products, including smart windows, rear-view mirrors, and protective glass for museum display cases. Electrochromic devices are devices that change light (and heat) transmission properties in response to voltage applied across the device. Electrochromic devices can be fabricated which electrically switch between transparent and translucent states (where the transmitted light is colored). Furthermore, certain transition metal hydride electrochromic devices can be fabricated which switch between transparent and reflective states. A more detailed discussion of the functioning of electrochromic devices is found in Granqvist, C.-G., Nature Materials, v5, n2, February 2006, p 89-90. Electrochromic devices are currently the most promising electrically tintable devices for use in smart windows.
SPDs and PDLC devices have also been incorporated into smart windows. SPDs are devices which have a thin film containing a suspension of numerous microscopic particles and transparent electrodes on either side of the film. The particles are randomly oriented and reduce the transmittance of the film. However, when an electric field is applied across the film, the particles align with the field, increasing the optical transmittance of the film. PDLCs comprise a liquid crystal layer sandwiched between transparent conductors on a thin plastic film. The liquid crystal particles are randomly oriented in the layer and scatter light—the layer is translucent. However, when a field is applied across the liquid crystal layer, the crystals are aligned to provide an optically transparent film. The degree of transparency is controlled by the voltage applied across the liquid crystal layer.
When electrochromic devices are incorporated in smart windows there is a need for the electrochromic devices to have a guaranteed lifetime of at least ten years and preferably thirty years or more. However, exposure of the electrochromic devices to atmospheric oxygen and water can degrade the performance of the devices and reduce the lifetime of the devices. Incorporation of the electrochromic device on an interior surface of an insulated glass unit (IGU) may provide the protected environment required for satisfactory performance over thirty years or more. SPDs and PDLC devices also benefit from the environmental protection afforded by an IGU.
Smart windows are generally in the form of an IGU. An IGU 100 including an electrically tintable device 220 is shown in FIGS. 1-3. Controller and power supply 190 is attached to the electrically tintable device 220 by electrical leads 195. FIG. 3 shows a cross section of the IGU, not to scale, which is comprised of two pieces of glass 210 and 330 spaced apart by a frame 340 along all four edges and sealed together along all four edges by sealant 350. The interior volume 380 is filled with an inert gas, such as argon, so as to provide thermal insulation and a non-oxidizing ambient. When an electrically tintable device 220 is incorporated into the IGU it is fabricated on the exterior piece of glass 210 (the outdoors facing piece) and is positioned on the interior facing surface thereof. See FIG. 2, which is a top view of the interior surface of the exterior piece of glass 210. The inert environment within the IGU does not affect the performance of an electrically tintable device 220.
The exterior piece of glass 210 in an IGU may be tinted, laminated, tempered, heat strengthened, etc. When an electrochromic device 220 is incorporated into an IGU as described above, the exterior piece of glass 210 will be subject to thermal stress. Furthermore, if the exterior piece of glass 210 is also tinted this will increase the thermal stress in the glass. Consequently, tempered or heat strengthened glass is often specified and needed.
Currently, the underlying philosophy for manufacturing smart windows including electrically tintable devices is to incorporate all glass functionalities—tintable device, color/tint, thickness, strength, size, etc. into a single sheet of glass. However, this philosophy leads to an inflexible and costly manufacturing process, with a limited range of integratable functionalities, which are a consequence of the conflicting manufacturing requirements for the different functionalities. For example, consider the problems with a manufacturing process for smart windows with tempered glass and an electrically tintable electrochromic device, where a range of window sizes is desired. The processing requirements for the two features—tempered glass and an electrochromic device—are incompatible with an integrated process including glass sizing. This is because after the glass has been tempered it cannot be cut to produce different size pieces, and if the electrochromic device is deposited on the glass prior to tempering, then the electrochromic device cannot withstand the process required to temper the glass. Therefore, the glass must be cut and tempered prior to deposition of the electrochromic device layers. This results in the requirement for the manufacturing equipment to handle a variety of glass sheet sizes, which results in costly manufacturing equipment and processes. Furthermore, handling a variety of different substrate sizes may have a negative impact on yield. For example, different sizes of glass sheet will present a different electric field distribution in a deposition chamber which may negatively impact the uniformity of deposited layers. (This is significant in deposition techniques involving ionized species—for example, plasma enhanced deposition.) Another example is that of combining different glass thicknesses with deposition of an electrochromic device. When depositing the layers of an electrochromic device, as for most typical thin film deposition processes, control of the substrate temperature is desirable; however, different thicknesses of glass will heat and cool at different rates, leading to lower throughput for thicker glass and to difficulties in controlling process uniformity. Clearly, there is a need for manufacturing processes for smart windows that are more flexible and compatible with lower cost, high yield and high throughput manufacturing.